Process to produce nonwoven fabrics with totally or partially hydrophilic areas and hydrophobic areas

ABSTRACT

A nonwoven fabric with totally or partially hydrophilic areas and hydrophobic areas which have a certain geometry and, optionally, a hydrophilic gradient is provided. The process to manufacture the fabric includes a treatment stage thereof with a plasma, whether in a vacuum or at atmospheric pressure. By controlling the plasma generation and application parameters, the particular area and the quantity of hydrophilic groups incorporated in the fabric surface is controlled. The fabric can be used in applications of the hygiene and sanitary sector to manufacture nappies, incontinence products and feminine hygiene products, for example.

RELATED APPLICATIONS

This application claims priority to Spanish Application No. 200502891,filed Nov. 23, 2005, which is incorporated herein by reference.

DESCRIPTION

1. Field of the Invention

The present invention belongs to the field of nonwoven fabricsmanufactured with polyolefins which, by their very nature, arehydrophobic, i.e. they have a degree of water repellency. Morespecifically, it relates to a process to produce nonwoven fabrics whichhave totally or partially hydrophilic areas and hydrophobic areas with acertain geometry and, optionally, with a hydrophilic gradient. Saidfabrics can be used as the top sheet in different hygiene products asthey have advantages with respect to a feeling of dryness and comfort.

2. Background of the Invention

Many applications from the hygiene sector need a hydrophilic fabric.Thus, in most nappies, incontinence products and feminine hygieneproducts currently manufactured, the part called top sheet, which comesinto contact with urine and other secretions is hydrophilic.

Currently, the treatment performed to convert a hydrophobic nonwovenfabric into a hydrophilic nonwoven fabric uses superficial treatmentswith hydrophilic agents or wetting solutions sometimes calledsurfactants.

Surfactants are substances which are defined from a physicochemicalpoint of view as a polar-nonpolar duality. The polar part has affinityfor polar solvents, in particular for water and is called hydrophilic.By contrast, the nonpolar group is called the hydrophobic or lipophilicpart.

When the nonwoven fabric undergoes treatment with surfactant solutions,the fabric only contains the active part of the solution once it hasdried, i.e. the surfactant. In the case of a nonwoven fabricmanufactured with polypropylene, the hydrophobic part of the surfactantis oriented towards the polypropylene chains (100% nonpolar) and thehydrophilic part towards the exterior of the polypropylene chainsincreasing the critical surface tension of the polypropylene, giving thenonwoven fabric hydrophilic characteristics.

When a drop of liquid comes into contact with a solid, different forcesact on it: the surface tension of the liquid, the critical surfacetension of the solid and the interfacial tension between both compounds.The ratio between these forces will depend on the greater or lesserresistance of the fabric to the passage of water or another liquid.

Therefore, from these three parameters, two of them (critical surfacetension of the solid and interfacial tension between fabric and liquid)directly depend on the raw material.

The critical surface tension of polypropylene is approximately 29 mN/m,whilst that of pure water is 72 mN/m. The smaller the difference(increasing the value of the critical surface tension of polypropylene),the greater the absorbency produced.

Nevertheless, the use of solutions with liquid hydrophilic agents hassome drawbacks:

1. Important generation of wastes that must be treated. This representshigh handling and treatment costs.

2. As the nonwoven fabric is treated with a solution, this involves thesubsequent drying of the water and, therefore, energy consumption.

3. The solutions cannot be stored and reused due to the risk ofbacteriological contamination and the solution should be exhaustivelycleaned from the system when changing from one agent to another. Thisall represents an added cost and a risk from possible bacteriologicalcontamination.

An alternative technology to achieve fabrics with hydrophiliccharacteristics is plasma technology. The application of plasmatechnology to textiles commenced in Russia in the 1960s, taking centerstage in the United States and Europe from the 1980s. Several studieshave been published since then with the results of experiments in vacuumreactors basically designed for the treatment of inorganic materials.

The advantages of these new plasma technologies compared with the humidmethod are evident:

1. Reduction or elimination of liquid hydrophilic agents and water.

2. Reduction or elimination of wastes generated by the use of liquidhydrophilic agents.

3. Less risk of bacterial contamination.

Nevertheless, the hydrophilic coatings produced are uniform.

Nonwoven fabrics have been disclosed in the state of the art withspecific hydrophilic areas, based on a series of openings made on thesurface therein (U.S. Pat. No. 6,911,573).

OBJECT OF THE INVENTION

The present invention, therefore, has the object of providing a nonwovenfabric with totally or partially hydrophilic areas and hydrophobic areaswith a defined geometry of the hydrophilic areas on its surface.

Another object of the invention is to provide a process to produce saidnonwoven fabric with totally or partially hydrophilic areas andhydrophobic areas.

Finally, another object of the invention is to provide the use of saidnonwoven fabric with totally or partially hydrophilic areas andhydrophobic areas in applications of the hygiene or sanitary sector.

DESCRIPTION OF THE INVENTION

In an aspect of the invention, a nonwoven fabric is provided withtotally or partially hydrophilic areas and hydrophobic areas which havea defined geometry of the hydrophilic areas on its surface.

In a particular embodiment, the hydrophilic areas have a hydrophilicgradient.

Thus, in the nonwoven fabric, the aim is to generate hydrophilic areasof different or equal degree, i.e. that more liquid passes through onone edge of the area than on the other. The areas that are or are notabsorbent, as well as the gradient areas, are not random.

There can be any geometry in accordance with the use that is going to bemade of said nonwoven fabric. Thus, hydrophilic areas can be produced inthe top sheet of a nappy where one wants liquid to pass through, i.e.hydrophilic, and other predetermined untreated areas so that said areasare hydrophobic.

In a particular embodiment, said nonwoven fabric is manufactured with apolyolefin. In a preferred embodiment, said nonwoven fabric ismanufactured with polypropylene. As has been mentioned, polypropyleneis, among many others, a hydrophobic organic polymer used to manufacturehydrophobic nonwoven fabrics, i.e. with a certain degree of waterrepellency.

In another particular embodiment, said nonwoven fabric is a spunbondfabric. In another particular embodiment, said nonwoven fabric is ameltblown fabric.

Spunbond fabric is formed from continuous fibers, which are more or lesssmooth, with a diameter in the order of 17-20 microns which cohere toone another by calendering. Thus, generating close soldering pointsjoined by portions of straight, and comparatively very hard, fibers,gives a certain rigidity to the fabric.

Meltblown fabric is formed from continuous fibers with very finediameters in the order of 4 to 7 microns which cohere to one anotherwithout the need for a final cohesion process (a calender or ahydroligand, for example).

In another aspect of the invention, a process is provided to produce anonwoven fabric with totally or partially hydrophilic areas andhydrophobic areas as previously described, which comprises a stage oftreating the fabric with gaseous plasma.

In the context of the invention, the term “plasma” relates to apartially ionized gas formed from ions, electrons and neutral species,capable of modifying the substrate surface without modifying thesubstrate's intrinsic properties.

The plasma is generated by applying a large quantity of energy to avolume of gas, the following parameters defining the finalcharacteristics of the surface to treat:

-   -   Gas composition    -   Plasma generation and application properties: pressure, power,        frequency, application time and application distance.    -   Substrate properties.

The final characteristics of the surface to treat that can be producedin accordance with the three previous points are varied: hydrophilic,hydrophobic, printing and sterilization, among others. The objective ofthis invention is to produce hydrophilic properties to a greater orlesser extent in certain fabric areas.

The substrate, as has already been mentioned, is a nonwoven fabric, ofspunbond or meltblown type, or the combination of both, manufacturedwith polyolefins, in general and polypropylene, in particular. In aparticular embodiment, said fabric is manufactured with polypropylene.

When the polypropylene is bombarded with plasma, a series of chemicaland/or physical reactions occur which modify the hydrophobic nature ofthe polypropylene.

In accordance with the gas used, the reactive species as well as theions will be different and, therefore, the action on the polypropylenewill give it different final characteristics. The gas used may be of adifferent nature, e.g. organic, inorganic or a liquid precursor. Themodification of the substrate by plasma treatment is superficial and theintrinsic characteristics of the polypropylene are not modified, i.e.the mechanical properties, e.g. tractions and elongations.

There are different forms of inducing the ionization of gases inaccordance with the technology applied:

-   -   Glow discharge (at low pressures, vacuum plasma).    -   Corona discharge (at atmospheric pressure or slightly higher).    -   Dielectric barrier discharge (at atmospheric pressure).

The source of gas ionization may be, for example, a two-electrode systemwhereby the electric energy of a radiofrequency generator is applied.

The objective of this invention is to produce hydrophiliccharacteristics using plasma technology in a vacuum or at atmosphericpressure. In the case of vacuum-plasma, greater uniformity andflexibility is attained than with any other plasma treatment.

The plasma is applied according to a variable geometry by controllingcertain plasma generation and application parameters. In this way, thearea and quantity of hydrophilic groups which are incorporated in thenonwoven fabric's surface is controlled.

The plasma generation and application parameters that are controlled arethe following:

-   the power or voltage of the gas ionization source to generate the    plasma;-   the frequency of the gas ionization to generate the gaseous plasma;-   the plasma application distance, and-   the plasma application time.

Depending on the degree and dimension of the hydrophilic areas one wantsto produce, it will act on either one or another concept or severalthereof.

Said parameters can be controlled, for example, by computer with the aidof suitable software.

Furthermore, via a plasma gradient on a spunbond or meltblown nonwovenfabric, for example, we can produce a greater or lesser deposition ofhydrophilic groups. It will depend on the quantity thereof that thenonwoven fabric is more or less hydrophilic in certain areas inaccordance with the gradient used.

The application of plasma by areas is provided to control the area andquantity of hydrophilic groups that are incorporated on the surface ofthe nonwoven fabric. There are different possibilities for theapplication of plasma by areas.

In a first embodiment, the plasma is projected from several individualheads, each one of which projects in a certain area, defining a certaingeometry on the nonwoven fabric. These heads have an on/off-typefrequency controller and enable partially hydrophilic areas to beproduced.

In a second embodiment the plasma is projected from a single headpositioned across the nonwoven fabric and, between said head and thesubstrate to treat, there is a sheet of stainless steel, aluminum oranother material provided with holes. Said holes can have variedgeometries and can be distributed on the surface of the sheet in verydifferent ways, so that the nonwoven fabric produced will havehydrophilic areas with a geometry defined by the projection of plasmathrough the holes in the sheet.

A third embodiment comprises a single head positioned throughout thefabric and incorporates software that permits defining the number ofplasma pulsations per time unit. This permits the intermittentapplication of plasma on the nonwoven fabric which will be displacedunder the head producing previously defined hydrophilic and hydrophobicareas in the end product.

Another group of solutions will focus on the treatment subsequent to theplasma treatment. This treatment consists of a deposition of a vaporizedmonomer together with hydrophilic groups. This gives properties thatlast with time, as the anchoring of the functional groups is muchstronger.

The use of plasma as pre-treatment means the activation of the nonwovenfabric's fibers for subsequent coating treatment by deposition of avaporized monomer with philic functional groups. Said deposition isperformed in the areas of the nonwoven fabric previously treated withplasma. It is necessary to provide the system with a vacuum box underthe nonwoven fabric to direct and collect the vapor generator. Aftervaporization, it is necessary to cure the monomer to produce homogeneoustreatment in the areas treated.

The deposition treatment can be performed using a monomer evaporator anda plate with holes positioned across the nonwoven fabric. Said holes canhave varied geometries and can be distributed on the surface of theplate in very different manners.

In another possible embodiment, the deposition treatment is performed byseveral evaporators using corresponding rotating cylindrical plates,which are individual, perforated and oriented towards the nonwovenfabric. Said holes can have varied geometries and may be distributed onthe surface of the plate in very different forms.

In either of the two cases, the nonwoven fabric that will be producedhas coated hydrophilic and hydrophobic areas in previously defined areasdistributed with a defined geometry.

Another aspect of the invention provides the use of said nonwoven fabricwith totally or partially hydrophilic areas and hydrophobic areas inapplications of the hygiene or sanitary sector. In a preferredembodiment, said fabric is used to manufacture a hygiene or sanitaryproduct, such as a nappy, an incontinence product or a feminine hygieneproduct. Said product is much more comfortable since the process of theinvention permits a top sheet design which keeps the skin in contactwith said top sheet dryer and more hygienic. Indeed, the hydrophilicareas of the hygiene product's top sheet, transport body fluids to theabsorbent core, achieving a feeling of dryness and comfort, and itavoids the skin being humid for too long, thus preventing problems dueto excess hydration of the skin or due to contact thereof with thebiological or chemical materials in the fluids.

BRIEF DESCRIPTION OF THE FIGURES

To complement the description being made and with the object of aidingtowards a better understanding of the characteristics of the invention,in accordance with a preferred example of embodiment thereof, a set ofdrawings is attached as an integral part of said description, whereinthe following has been represented, with an illustrative, non-limitingcharacter:

FIG. 1 shows a schematic representation of a plasma projection systemusing individual heads applied on a nonwoven fabric.

FIG. 2 shows a schematic representation of a plasma projection systemusing a single head applied across the fabric with the intermediation ofa sheet provided with holes.

FIG. 3 shows a schematic representation of a plasma projection systemusing a single head across the fabric, provided with software to definethe number of pulsations that is translated in plasma projections pertime unit.

FIG. 4 shows a schematic representation of a coating system bydeposition of a monomer using a plate, as well as showing a detailedview of the unequal structure produced in the nonwoven fabric.

FIG. 5 shows a possible monomer deposition coating system using monomerevaporators with perforated plates oriented towards the nonwoven fabric,as well as showing a detailed view of the unequal structure produced inthe nonwoven fabric.

FIG. 6 shows another detailed view of the unequal structure of thefabric produced by the process of projection by areas.

FIG. 7 shows another detailed view of the unequal structure of thefabric produced by the process of projection by areas.

PREFERRED EMBODIMENT OF THE INVENTION

Below, a preferred embodiment of the invention will be described withreference to the figures.

As is observed in FIGS. 1 to 3, the process to produce nonwoven fabricswith totally or partially hydrophilic areas and hydrophobic areas thatcomprises the object of this invention basically consists of the partialprojection of plasma on certain sectors of the surface of a nonwovenfabric (2) manufactured with a hydrophobic material, preferablypolyolefin, to produce in those sectors a geometry of hydrophilic areas(5) and/or partially hydrophilic areas (7) between hydrophobic areas(6).

FIGS. 1 to 3 show different possibilities with regard to the means usedfor plasma application.

These FIG. 1 shows several heads (1) that project plasma on a nonwovenfabric (2) in different sectors, whilst this continually moves.

FIG. 2 shows that the head (1) projects plasma throughout the width ofthe nonwoven fabric (2) using a sheet (3), provided with holes (4) to,in this way, define the hydrophilic areas (5) in the nonwoven fabric incorrespondence with said holes (4).

In FIG. 3, the nonwoven fabric (2) is displaced under a head (1) whichprojects plasma at different instants controlled by software.

The heads (1) can also be displaceable to define different geometries ofhydrophilic areas (5) or partially hydrophilic areas (7) on the nonwovenfabric (2).

The result of the unequal application of the plasma projection on thenonwoven fabric (2) causes a specific geometry, as is reflected in FIG.6, or as in the case of FIG. 7 which shows a nappy with hydrophilicareas (5), hydrophobic areas (6) or partially hydrophilic areas (7).

The heads (1) incorporate frequency controllers, not represented, whichenable the plasma projection frequency to be modified to produce thepartially hydrophilic areas (7) on the nonwoven fabric (2).

FIG. 4 represents a monomer evaporator (8) which is appliedcomplementarily to the plasma projection on the hydrophilic areas (5) ofthe nonwoven fabric (2) giving rise to coated hydrophilic areas (13).Said FIG. 4 represents a plate (9) provided with openings (10) wherebythe monomer passes to coat the hydrophilic areas (5).

Likewise, FIG. 5 has represented several monomer evaporators (8) incorrespondence with which are positioned rotating plates (11) providedwith openings (12) whereby the monomer is projected for its depositionon the hydrophilic areas (5) of the nonwoven fabric (2) giving rise tocoated hydrophilic areas (13).

1. A method of producing a nonwoven fabric with totally or partiallyhydrophilic areas and hydrophobic areas, which comprises projectingplasma on determined sectors of the surface of a nonwoven fabricmanufactured with a hydrophobic material, to produce, in those sectors,a geometry of hydrophilic areas and/or partially hydrophilic areasbetween hydrophobic areas.
 2. The method of claim 1, wherein thehydrophobic material is polyolefin.
 3. The method according to claim 1,wherein the plasma projection is performed in a vacuum.
 4. The methodaccording to claim 1, wherein the plasma projection is performed atatmospheric pressure.
 5. The method according to claim 1, furthercomprising applying a coating to the hydrophilic area or partiallyhydrophilic area by deposition of a vaporized monomer with functionalphilic groups after applying the plasma.
 6. The method according toclaim 5, further comprising performing a curing phase after coating. 7.The method according to claim 1, wherein the nonwoven fabric ispolypropylene.
 8. The method according to claim 1, wherein the nonwovenfabric is a meltblown fabric.
 9. The method according to claim 1,wherein the nonwoven fabric is a spunbond fabric.
 10. The methodaccording to claim 1, wherein the nonwoven fabric is a combined spunbondand meltblown fabric.
 11. A nonwoven fabric produced with totally orpartially hydrophilic areas and hydrophobic areas in accordance with theprocess described in claim
 1. 12. A hygiene or sanitary productcomprising a nonwoven fabric with totally or partially hydrophilic areasand hydrophobic areas according to claim
 11. 13. The hygiene or sanitaryproduct according to claim 12 which is selected from the groupconsisting of nappies, incontinence products and feminine hygieneproducts.
 14. The hygiene or sanitary product according to claim 12which is an absorbent product used in operating theatres.